Copper fiber nonwoven fabric

ABSTRACT

Provided is a copper fiber nonwoven fabric having a bonding portion between copper fibers, and in the relationship between compressive stress and strain, is provided with a first region exhibiting plastic deformation and a second region exhibiting elastic deformation in which compressive stress is higher than the first region. Alternatively, the copper fiber nonwoven fabric has a bonding portion between copper fibers, and in the relationship between compressive stress and strain, has a region exhibiting elastic deformation, wherein the region exhibiting elastic deformation has an elastic deformation region before an inflection portion a, the inflection portion a, and an elastic deformation region after the inflection portion a.

TECHNICAL FIELD

The present invention relates to a copper fiber nonwoven fabric obtained by bonding between copper fibers.

The present application claims priority on the basis of Japanese Patent Application No. 2016-187232 filed in Japan on Sep. 26, 2016, the contents of which are incorporated herein by reference.

BACKGROUND ART

Copper fibers have conventionally been processed into the form of sheets and are used in, or are being examined for use in, applications for filters or electromagnetic shielding and the like by taking advantage of their properties such as electrical conductivity, bactericidal activity and air permeability.

An example of such a filter application that has been proposed consists of a cylindrical metal filter, which enables the density and thickness of the filter body to be adjusted corresponding to the size of impurities to be filtered out present in a high-temperature gas, comprising an internal cylindrical wire mesh having an overlaying portion welded in the longitudinal direction, a filter body obtained by winding a felt sheet made of metal fiber around the internal cylindrical wire mesh at a prescribed thickness and impregnating and drying a heat-resistant resin, and an external cylindrical wire mesh having an overlaying portion welded in the longitudinal direction (see, for example, Patent Document 1).

Processing such as needle punching or pressing is presumed to be used to produce the aforementioned filter body, and metal fibers capable of plastic deformation are proposed to be used preferably for the material of the filter body. Copper fibers are also indicated as being able to be used for these metal fibers.

In addition, a filter device has been proposed that employs a sealing structure capable of preventing damage and so forth to a filtration member by installing an elastic member between a pressing member and the filtration member, and nickel, chromium, alloys thereof, stainless steel and titanium alloy are proposed as examples of metals that can be used for the elastic member (see, for example, Patent Document 2).

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Unexamined Patent Application, First Publication No. H9-276636

Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2004-305964

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

When stress is applied to metal, elastic deformation is known to typically occur followed by the occurrence of plastic deformation. “Elastic deformation” refers to deformation in the case of having applied an external force to an object causing deformation followed by a return to the original shape when that external force is removed. On the other hand, “plastic deformation” refers to deformation in which an external force is applied to an object causing deformation followed by that deformation remaining even after the external force is removed.

However, when metal was used as a material and the like by focusing on such metal properties of electrical conductivity, durability and toughness, the metal ended up being governed by the previously described property, namely the initial occurrence of elastic deformation followed by the occurrence of plastic deformation. In addition, the aforementioned elastic deformation accumulates stain in proportion to, for example, compressive stress. Thus, a material has been sought that, despite being a metal, initially exhibits plastic deformation followed by the occurrence of elastic deformation, which differs from the property typically demonstrated by metals, and has the property of undergoing a change in strain in response to, for example, compressive stress within the region exhibiting that elastic deformation. In particular, a sheet-like material has been sought that exhibits the aforementioned properties in the case of considering the degree of freedom of the shape thereof, such as being able to be arranged in confined locations.

However, in Patent Document 1, a filter body is used that is composed of unbound metal fibers. In addition, although the use of metal fibers capable of plastic deformation is described in Patent Document 1, there is no clear description of the intent thereof or a description regarding elastic deformation.

Patent Document 2 proposes that sealability is attempted to be enhanced by utilizing the compressive elastic modulus of a support body 2B and an elastic member, while there is no description relating to plastic deformation. In addition, there is no description relating to a change in strain within a region of elastic deformation. Moreover, although Patent Document 2 does not contain any description or suggestion regarding copper fibers, this is because copper fibers are not preferable for use as metal capable of satisfying the requirements of strength, corrosion resistance, heat resistance and toughness in elastic member applications.

In contrast, the copper fiber nonwoven fabric of the present invention has a bonding portion between copper fibers and, in the relationship between compressive stress and strain, is provided with a first region exhibiting plastic deformation and a second region exhibiting elastic deformation in which compressive stress is higher than in the first region.

Alternatively, the copper wire nonwoven fabric of the present invention has a bonding portion between copper fibers and, in the relationship between compressive stress and strain, has a region exhibiting elastic deformation, wherein the region exhibiting elastic deformation has an elastic deformation region before an inflection portion a, the inflection portion a, and an elastic deformation region after the inflection portion a.

As a result thereof, an object of the present invention is to provide a copper fiber nonwoven fabric that exhibits cushioning while also having high shape followability.

Means for Solving the Problems

As a result of conducting extensive studies, the inventors of the present invention found that, by providing a bonding portion between copper fibers, a copper fiber nonwoven fabric has, in the relationship between compressive stress and strain, a first region exhibiting plastic deformation and a second region exhibiting elastic deformation in which compressive stress is higher than in the first region. Alternatively, the inventors of the present invention found that, by providing a bonding portion between copper fibers, a copper fiber nonwoven fabric has, in the relationship between compressive stress and strain, a region exhibiting elastic deformation, wherein the region exhibiting elastic deformation has an elastic deformation region before an inflection portion a, the inflection portion a, and an elastic deformation region after the inflection portion a, thereby leading to invention of the present copper fiber nonwoven fabric.

Namely, the present invention is as described below.

(1) A copper fiber nonwoven fabric having a bonding portion between copper fibers and, in the relationship between compressive stress and strain, is provided with a first region exhibiting plastic deformation and a second region exhibiting elastic deformation in which compressive stress is higher than the first region.

(2) The copper fiber nonwoven fabric described in (1), having an inflection portion a in the second region exhibiting elastic deformation where strain exhibits inflection in response to compressive stress.

(3) A copper fiber nonwoven fabric having a bonding portion between copper fibers and, in the relationship between compressive stress and strain, has a region exhibiting elastic deformation, wherein the region exhibiting elastic deformation has an elastic deformation region before an inflection portion a, the inflection portion a, and an elastic deformation region after the inflection portion a.

Effects of the Invention

The copper fiber nonwoven fabric of the present invention, in the relationship between compressive stress and strain, has a first region exhibiting plastic deformation and a second region exhibiting elastic deformation in which compressive stress is higher than the first region. Alternatively, the copper fiber nonwoven fabric of the present invention has a region exhibiting elastic deformation, wherein the region exhibiting elastic deformation has an elastic deformation region before an inflection portion a, the inflection portion a, and an elastic deformation region after the inflection portion a. As a result, the copper fiber nonwoven fabric is able to demonstrate favorable cushioning while having shape followability.

Namely, in the case the copper fiber nonwoven fabric of the present invention is subjected to compressive stress, the copper fiber nonwoven fabric is able to follow the shape of the compressed object due to the first region exhibiting plastic deformation and the elastic deformation region before the inflection portion a, and demonstrate favorable cushioning due to the second region exhibiting elastic deformation and the elastic deformation region after the inflection portion a.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photomicrograph showing an SEM cross-section of a copper fiber nonwoven fabric.

FIG. 2 is a photomicrograph showing the state in which copper fibers are bonded.

FIG. 3 is a graph obtained during measurement of the relationship between compressive stress and strain in the copper fiber nonwoven fabric of the present invention (one embodiment).

FIG. 4 is a graph for specifically explaining a region exhibiting elastic deformation according to the present invention (second region exhibiting elastic deformation).

FIG. 5 is a graph obtained during measurement of the relationship between compressive stress and strain in the copper fiber nonwoven fabric of another embodiment of the present invention.

FIG. 6 is a graph obtained during measurement of the relationship between compressive stress and strain in the copper fiber nonwoven fabric of another embodiment of the present invention.

FIG. 7 is a graph obtained during measurement of the relationship between compressive stress and strain in the copper fiber nonwoven fabric of another embodiment of the present invention.

FIG. 8 is a graph obtained during measurement of the relationship between compressive stress and strain in the copper fiber nonwoven fabric of another embodiment of the present invention.

FIG. 9 is a graph obtained during measurement of the relationship between compressive stress and strain in the copper fiber nonwoven fabric of another embodiment of the present invention.

FIG. 10 is a graph obtained during measurement of the relationship between compressive stress and strain of a copper plate.

FIG. 11 is a schematic diagram of a pressing device for confirming cushioning of the copper fiber nonwoven fabric of the present invention.

FIG. 12 is a schematic diagram for explaining the details of a compressed body placed in a pressing device.

FIG. 13 is a photomicrograph indicating the status of a pressure-sensitive sheet after a press test of a copper fiber nonwoven fabric of Example 1.

FIG. 14 is a photomicrograph indicating the status of a pressure-sensitive sheet after a press test of a copper fiber nonwoven fabric of Example 5.

FIG. 15 is a photomicrograph indicating the status of a pressure-sensitive sheet after a press test of copper foil of Comparative Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

Although the following provides a detailed explanation of the copper fiber nonwoven fabric of the present invention, embodiments of the copper fiber nonwoven fabric of the present invention are not limited thereto.

In the present description, a nonwoven fabric refers to a sheet-like material obtained by randomly entangling fibers, and a copper fiber nonwoven fabric refers to a nonwoven fabric at least containing fibers composed of copper. The copper fiber nonwoven fabric of the present invention may be composed only of copper fibers or may have a component other than the aforementioned copper fibers. A bonding portion between the copper fibers refers to a site where the copper fibers are physically fixed in position. The copper fibers may be directly fixed each other, may be fixed each other by a second metal component having a metal component differing from the metal component of the copper fibers, or a part of the copper fibers may be fixed each other by a component other than the metal component. Among these, the copper fibers are preferably fixed directly from the viewpoints of ease of imparting favorable shape followability and cushioning to the copper fiber nonwoven fabric. FIG. 1 is photomicrograph showing an SEM cross-section of a nonwoven fabric composed only of copper fibers that was fabricated using copper fibers. In addition, FIG. 2 is a photomicrograph showing one example of the state of bonding between copper fibers.

Although there are no particular limitations on the aforementioned second metal component, examples thereof include stainless steel, iron, copper, aluminum, bronze, brass, nickel and chromium, and may also be a precious metal such as gold, platinum, silver, palladium, rhodium, iridium, ruthenium or osmium.

Polyolefins such as polyethylene terephthalate (PET) resin, polyvinyl alcohol (PVA), polyethylene or polypropylene, polyvinyl chloride resin, aramid resin, nylon, acrylic resin and organic materials having bonding properties and support properties of these fibrous materials can be used in those portions containing bonding portions as the aforementioned component other than the metal component.

Although the average fiber diameter of the copper fibers according to the present invention can be set arbitrarily within a range that does not impair the formation of the nonwoven fabric, it is preferably 1 μm to 70 μm and more preferably 15 μm to 30 μm. If average fiber diameter is less than 1 μm, the copper fibers tend to easily form so-called clumps when forming into the nonwoven fabric, while if average fiber diameter exceeds 70 μm, the rigidity of the copper fibers has the risk of acting to prevent fiber entanglement. In addition, although the cross-sectional shape of the copper fibers may be any of circular, elliptical, roughly quadrangular or irregular, at least copper fibers having a circular cross-section are preferably contained. Copper fibers having a circular cross-section bend more easily (bending portions) in response to stress than copper fibers having a prismatic cross-section, and since a difference in the degree of bending of the copper fibers is not likely to occur with respect to those locations subjected to stress, degree of bending also tends to be uniform. Suitable entanglement of copper fibers having bending portions tends to facilitate enhancement of shape followability and cushioning. Here, the circular cross-section is not required to be perfectly circular, but rather is only required to be a circular cross-section of a degree that facilitates the formation of suitable bending portions when subjected to stress generated in the production of ordinary copper fiber nonwoven fabric. Furthermore, in the present description, “average fiber diameter” refers to the average value (such as the average value of 20 fibers) of area diameter derived by calculating (by using known software, for example) the cross-sectional area of the copper fibers based on a vertical cross-section at an arbitrary location of the copper fiber nonwoven fabric photographed with a microscope, and calculating a diameter of a circle which has the same area as the aforementioned cross-sectional area.

The length of the copper fibers according to the present invention is preferably within the range of 1 mm to 50 mm and more preferably within the range of 3 mm to 20 mm. If the average fiber length is within the aforementioned ranges, in the case of, for example, producing the copper fiber nonwoven fabric by papermaking, in addition to making it difficult for so-called clumping of the copper fibers to occur and being able to expect the effect of facilitating suitable adjustment of dispersion, it also becomes easy to demonstrate the effect of improving sheet strength attributable to entanglement of the copper fibers. “Average fiber length” in the present description refers to the value obtained by measuring, for example, 20 fibers with a microscope and averaging the measured values. In addition, “papermaking” refers to the production of paper and the like by forming the raw material into a wet, pulpy material and thinly spreading followed by drying.

Although the thickness of the copper fiber nonwoven fabric can be adjusted to an arbitrary thickness, it is preferably, for example, within the range of 50 μm to 1.5 mm and more preferably 150 μm to 350 μm. Furthermore, “sheet thickness” in the present description refers to average value in the case of, for example, measuring thickness at several arbitrary measurement points of the copper fiber nonwoven fabric with a terminal drop type film thickness gauge using air (such as the ID-C112X Digimatic Indicator manufactured by Mitutoyo Corp.).

The space factor of the copper fiber nonwoven fabric of the present invention is within the range of 5% to 60%, preferably 5% to 40% and more preferably 10% to 25%. In the case the space factor is less than 5%, the amount of fiber becomes insufficient, thereby resulting in the risk in a decrease in shape followability and cushioning, while if the space factor exceeds 60%, the copper fiber nonwoven fabric becomes rigid resulting in the risk of a decrease in shape followability and cushioning. Space factor in the present description refers to the percentage of the portion where fibers are present to the volume of the copper fiber nonwoven fabric, and is calculated according to the equation below from the basis weight and thickness of the copper fiber nonwoven fabric as well as the true density of the copper fibers (case of copper fiber nonwoven fabric composed only of copper fibers).

Space factor (%)=(basis weight of copper fiber nonwoven fabric)/(thickness of copper fiber nonwoven fabric×true density of copper fibers)×100

The following provides an explanation of the case of the copper fiber nonwoven fabric of the present invention not being subjected to external force following fabrication.

Plastic deformation and elastic deformation of the copper fiber nonwoven fabric of the present invention can be confirmed from a stress-strain curve by carrying out a compression test using a cycle consisting of compression and release. Namely, since the copper fiber nonwoven fabric of the present invention undergoes plastic deformation due to the operation of the first compression and release and the copper fiber nonwoven fabric undergoes a reduction in thickness during the second compression, the starting point of strain (starting position of the compression probe) also lowers beyond that at the time of the absence of compression. (For example, in the graph shown in FIG. 3, since the starting point of the horizontal axis is the point where plastic deformation of the uncompressed copper fiber nonwoven fabric starts, strain starting from the second time is indicated with a positive value.) In the present description, the low strain side is defined as a plastic deformation region (first region exhibiting plastic deformation), and strain beyond the plastic deformation region (high strain side) is defined as an elastic deformation region (second region exhibiting elastic deformation or region indicating elastic deformation) bordering on the strain starting value (compression probe starting position) at the time of existing compression (during compression for the second or third time) in the compression test. Furthermore, the plastic deformation region on the low strain side is not required to be composed only of plastic deformation, but rather elastic deformation may also be present in the plastic deformation region of the low strain side in addition to plastic deformation.

FIG. 3 is a graph obtained during measurement of the relationship between compressive stress and strain in the copper fiber nonwoven fabric of the present invention described in Example 1 of the present description. In the graph, 1st time to 3rd time indicates the number of times compression was applied, the plot of the 1st time indicates measured value during initial compression, the plot of the 2nd time indicates measured values during the second compression, and the plot of the 3rd time indicates measured values during the third compression. According thereto, the copper fiber nonwoven fabric of the present invention can be determined to have a first region A exhibiting plastic deformation, and a second region B exhibiting elastic deformation where compression stress is higher than in the aforementioned first region. Namely, in contrast to the first region A exhibiting plastic deformation being absent in the measured values of the copper plate of FIG. 10, in the copper fiber nonwoven fabric of the present invention shown in FIG. 3, for example, plastic deformation appears as a first region followed by the appearance of elastic deformation as a second region.

Moreover, the second region B exhibiting elastic deformation preferably has an inflection portion a where strain exhibits inflection in response to compressive stress. The “inflection portion a” refers to the portion where the slope of a tangent changes rapidly in a stress-strain curve. FIG. 4 is a graph for specifically explaining the second region exhibiting elastic deformation according to the present invention (region exhibiting elastic deformation). The data values are the same as those of FIG. 3. A region B1 exhibiting elastic deformation before the inflection portion a shown in FIG. 4 (located between region A and inflection portion a) is understood to be a so-called spring elasticity region, while a region B2 exhibiting elastic deformation after the inflection portion a is understood to be a so-called strain elasticity region where strain is accumulated within the metal. Namely, the copper fiber nonwoven fabric of the present invention has region B1 exhibiting elastic deformation before the inflection portion a and a region B2 exhibiting elastic deformation after the inflection portion a in the second region B exhibiting elastic deformation (region exhibiting elastic deformation). As a result, the effect is demonstrated of facilitating enhancement of shape followability and cushioning. Furthermore, the inflection portion a and the region B1 exhibiting elastic deformation before the inflection portion a are not present in the measured values of the copper plate of FIG. 10.

Next, an explanation is provided of the case of the copper fiber nonwoven fabric already having been subjected to external force following fabrication.

In the case of the copper fiber nonwoven fabric of the present invention having already been subjected to external force (such as compressive stress) following the fabrication thereof, the aforementioned first region exhibiting plastic deformation is basically no longer present (at the stage the first cycle of compression and release has ended in terms of the example of FIG. 4). However, the copper fiber nonwoven fabric of the present invention has an elastic deformation region before the inflection portion a, the inflection portion a, and an elastic deformation region after the inflection portion a in the region exhibiting elastic deformation as previously described, even if a region exhibiting plastic deformation is absent. As a result, shape followability and cushioning are demonstrated that are superior to those of typical metals. In addition, a first region exhibiting plastic deformation ought to be present during fabrication of the copper fiber nonwoven fabric of the present invention even in the case of already having been subjected to external force.

The method used to measure the relationship between compressive stress and strain in the present description uses a tension-compression strain measuring tester. First, a square test piece measuring 30 mm on a side is prepared. The thickness of the prepared test piece is measured as the thickness before compression testing using the ID-C112X Digimatic Indicator manufactured by Mitutoyo Corp. This micrometer is able to raise and lower the probe using air. In addition, the speed thereof can also be set arbitrarily. Since the test piece is in a state of being easily crushed by a small amount of stress, when lowering the measuring probe, lower as slowly as possible so that only the dead weight of the probe acts on the test piece. Moreover, the probe is contacted with the test piece only once. The thickness measured at that time is taken to be the “pre-test thickness”.

Continuing, the compression test is carried out using the test piece. A 1 kN load cell is used. A compression probe made of stainless steel and having a diameter of 100 mm is used for the jig used in the compression test. The compression rate is set to 1 mm/min and the compression and release operations are consecutively carried out three times on the test piece. As a result, plastic deformation and elastic deformation of the copper fiber nonwoven fabric of the present invention can be confirmed.

In addition, actual strain in response to stress can be calculated from the stress-strain curve obtained according to the compression test, and the amount of plastic deformation can be calculated in accordance with the equation below.

Amount of plastic deformation=(strain of first rising portion of curve)−(strain of second rising portion of curve)

At this time, a rising portion refers to strain at 2.5 N.

Thickness of the test piece after testing is measured using the same method as previously described and this is taken to be the “post-test thickness”.

In addition, the plastic deformation rate of the copper fiber nonwoven fabric of the present invention is preferably within a desired range. Plastic deformation rate refers to the degree of plastic deformation of the copper fiber nonwoven fabric. Plastic deformation rate (such as the plastic deformation rate when gradually increasing the load from 0 MPa to 1 MPa) is defined as indicated below.

Amount of plastic deformation (μm)=T0−T1

Plastic deformation rate (%)=(T0−T1)/T0×100

The aforementioned T0 is the thickness of the copper fiber nonwoven fabric prior to the application of a load, while the aforementioned T1 is the thickness of the copper fiber nonwoven fabric after releasing the load after it has been applied.

The copper fiber nonwoven fabric of the present invention is required to have a region where plastic deformation occurs as a first region or have an elastic deformation region before an inflection portion a, the inflection portion a, and an elastic deformation region after the inflection portion a in a region exhibiting elastic deformation. In the case of the first region where plastic deformation occurs, the plastic deformation rate is preferably 1% to 90%, more preferably 4% to 75%, even more preferably 30% to 60% and most preferably 47% to 60%. As a result of the plastic deformation rate being 1% or more, shape followability attributable to plastic deformation can be preferably secured, thereby making this preferable. On the other hand, as a result of the plastic deformation rate being 90% or less, plastic deformation does not become excessive, and as a result of leaving an adequate margin for elastic deformation, cushioning attributable to elastic deformation can be secured, thereby making this preferable.

Elongation percentage of the copper fiber nonwoven fabric of the present invention is preferably within the range of 3% to 20%, more preferably 3% to 10%, and even more preferably 5% to 10%. If elongation percentage is less than 3%, there is the risk of a decrease in shape followability in cases, for example, of the surface of the followed object not being flat. If elongation percentage exceeds 20%, there is the risk of a decrease in shape stability of the nonwoven fabric.

Tensile strength of the copper fiber nonwoven fabric of the present invention is preferably 2 N/mm to 20 N/10 mm, more preferably 2 N/10 mm to 10 N/10 mm, and even more preferably 5 N/10 mm to 10 N/10 mm. If tensile strength is less than 2 N/10 mm, there is the risk of breakage of the copper fiber nonwoven fabric depending on the manner in which it is used, while if tensile strength exceeds 20 N/10 mm, there is the risk of a decrease in shape followability.

Clark stiffness (JIS P 8143:2009) of the copper fiber nonwoven fabric of the present invention is preferably 3 to 15, more preferably 3 to 12 and even more preferably 6 to 12. Although Clark stiffness may be 3 or less, there is the risk of increased susceptibility to wrinkling and the like from the viewpoint of handling of the copper fiber nonwoven fabric. In addition, if Clark stiffness is 15 or more, there is the risk of the occurrence of buckling depending on such factors as the shape and diameter of the followed object. The method used to measure Clark stiffness consists of a measurement method in which self-weight deflection is used as an indicator, and a large measured value indicates the strength of the so-called flexural rigidity of a sample. Thus, if the measured value of a sheet-like material is within a fixed range, balance is maintained between ductility and flexural rigidity and the followability of a followed object having a curved portion can be said to be superior.

Although there are no particular limitations on the sheet resistance value of the copper fiber nonwoven fabric of the present invention, it is preferably 0.8 mΩ/□ to 1.5 mΩ/□. Sheet resistance value can be determined according to, for example, the Van der Pauw method.

(Fabrication of Copper Fiber Nonwoven Fabric)

The copper fiber nonwoven fabric of the present invention can be obtained by, for example, a method consisting of compression molding copper fibers or a web consisting mainly of copper fibers, or papermaking using a wet papermaking method using copper fibers or a raw material consisting mainly of copper fibers.

In the case of obtaining the copper fiber nonwoven fabric of the present invention by compression molding, copper fibers or a web consisting mainly of copper fibers obtained by a card method or air-laid method can be compression-molded. At this time, a binder may be impregnated between the fibers in order to impart bonds between the fibers. There are no particular limitations on the binder, and examples of binders that can be used include organic binders such as acrylic adhesives, epoxy adhesives or urethane adhesives, and inorganic binders such as colloidal silica, water glass or sodium silicate. Instead of impregnating with binder, the surface of the fibers may be pre-coated with a thermoadhesive resin followed by applying pressure and heated compression after having laminated the copper fibers or an aggregate consisting mainly of copper fibers.

In addition, the copper fiber nonwoven fabric of the present invention can also be fabricated by a wet papermaking method in which copper fibers and the like are dispersed in water and then thinly spreading out the aqueous dispersion and drying. More specifically, copper fibers or slurry consisting mainly of copper fibers can be prepared followed by the suitable addition thereto of, for example, a filler, dispersant, thickener, antifoaming agent, paper strength enhancer, sizing agent, flocculant, colorant or fixing agent. In addition, organic fibers demonstrating adhesiveness by heating and melting in the manner of polyolefins such as polyethylene terephthalate (PET) resin, polyvinyl alcohol (PVA), polyethylene or polypropylene, polyvinyl chloride resin, aramid resin, nylon or acrylic resin, can be added to the slurry as fibrous materials other than copper fibers. For example, in the case of providing a bonding portion between the copper fibers by sintering, the absence of organic fibers and the like between the copper fibers more reliably facilitates the providing of a binding portion (by making it easier to increase the number of bonding points), and facilitates the obtaining of a first region exhibiting plastic deformation, a second region exhibiting elastic deformation in which compressive stress is higher than in the first region, and an elastic deformation region before an inflection portion a, the inflection portion a and an elastic deformation region after the inflection portion a in a region exhibiting elastic deformation. Consequently, the absence of organic fibers and the like between the copper fibers is preferable from the viewpoints of making it easier to impart shape followability and cushioning to the copper fiber nonwoven fabric of the present invention.

In the case of producing the copper fibers by a papermaking method in the absence of organic fibers and the like as previously described, aggregates such as so-called clumps form easily due to the difference in true density between water and the copper fibers and excessive entanglement of the copper fibers. Consequently, it is preferable to use a suitable thickener and the like. In addition, in the case of slurry present in a stirring mixer, copper fibers of high true density tend to easily precipitate to the bottom of the mixer. Consequently, slurry other than near the bottom of the mixer where the ratio of copper fibers is comparatively stable is preferably used for the slurry. Carrying out this procedure demonstrates the effect of facilitating the imparting of more accurate shape followability and cushioning with little in-plane variation.

Next, wet papermaking is carried out with a papermaking machine using the aforementioned slurry. A cylinder paper machine, Fourdrinier paper machine, tanmo machine, inclined paper machine or combination paper machine combining different types or the same types of these paper machine can be used as papermaking machines. Following papermaking, the wet paper can be dehydrated and dried using an air dryer, cylinder dryer, suction drum dryer or infrared dryer and the like to obtain a sheet.

In addition, when dehydrating, it is preferable to make the flow rate of water generated during dehydration (amount of water generated during dehydration) to be uniform within the plane or in the direction of width and so forth of the papermaking wire. Maintaining a constant flow of dehydration water facilitates the obtaining of a highly homogeneous copper fiber nonwoven fabric since disturbances during dehydration are suppressed and the rate at which the copper fibers precipitate into the papermaking wire is uniform. In order to maintain a constant flow rate during dehydration, measures can be adopted such as removing any structures having the potential to obstruct water flow below the papermaking wire. This demonstrates the effect of making it easy to impart the copper fiber nonwoven fabric with more accurate shape followability and cushioning with little in-plane variation.

When using a wet papermaking method, it is preferable to produce the copper fiber nonwoven fabric by going through a fiber entanglement treatment step in which copper fibers or components consisting mainly of copper fibers, which form a sheet containing water on the wire, are mutually entangled. Here, the fiber entanglement treatment step preferably employs, for example, a fiber entanglement treatment step in which a high-pressure jet stream of water is sprayed onto the surface of the wet material sheet. More specifically, as a result of arranging a plurality of nozzles in the direction perpendicular to the direction of sheet flow and spraying a high-pressure jet stream of water from the plurality of nozzles at the same time, the copper fibers or fibers consisting mainly of copper fibers are entangled over the entire sheet. After having gone through the aforementioned step, the wet material sheet goes through a drying step and is wound up.

The copper fiber nonwoven fabric of the present invention fabricated according to the aforementioned process may also be subjected to, for example, a pressing (pressurizing) step prior to bonding the copper fibers. By carrying out a pressing step prior to bonding, bonding portions can be easily provided (the number of bonding points can be easily increased) more reliably in the subsequent bonding step, and in addition to a first region exhibiting plastic deformation and a second region exhibiting elastic deformation in which compressive stress is higher than the aforementioned first region, an elastic deformation region before an inflection portion a, the inflection portion a and an elastic deformation region after the inflection portion a are easily obtained in the region exhibiting elastic deformation. Consequently, it is preferably to carry out the pressing step prior to bonding from the viewpoints of making it easy to impart shape followability and cushioning to the copper fiber nonwoven fabric of the present invention. In addition, although pressing may be carried out in the presence of heating or in the absence of heating, in the case the copper fiber nonwoven fabric of the present invention contains organic fibers and the like that demonstrating bonding properties as a result of heating and melting, it is effective to heat the fibers to a temperature equal to or higher than the melt starting temperature thereof, while in the case the copper fiber nonwoven fabric is composed by containing copper fibers alone or a second metal component, the fibers are only required to be pressed. Moreover, although the pressure applied during pressing may be suitably set in consideration of the thickness of the copper fiber nonwoven fabric, for example, in case of the copper fiber nonwoven fabric having a thickness of about 170 μm, by carrying out pressing at a linear pressure of less than 300 kg/cm and preferably less than 250 kg/cm, shape followability and cushioning are easily imparted to the copper fiber nonwoven fabric of the present invention, thereby making this preferable. In addition, this pressing step makes it possible to adjust the space factor of the copper fiber nonwoven fabric.

Examples of methods that can be used to bond copper fibers of a copper fiber nonwoven fabric prepared in this manner include a method consisting of sintering the copper fiber nonwoven fabric, a method consisting of bonding by chemical etching, a laser fusion method, a bonding method using induction heating (IH), a chemical bonding method and a thermal bonding method. Among the aforementioned methods, a method consisting of sintering the copper fiber nonwoven fabric is preferable from the viewpoints of easily obtaining a first region exhibiting plastic deformation and a second region exhibiting elastic deformation in which compressive stress is higher than the aforementioned first region, as well as an elastic deformation region before an inflection portion a, the inflection portion a and an elastic deformation region after the inflection portion a in the region exhibiting elastic deformation, while also facilitating the imparting of shape followability and cushioning to the copper fiber nonwoven fabric of the present invention. FIG. 2 is a photomicrograph observed by SEM of a cross-section of a copper fiber nonwoven fabric in which the copper fibers are bonded by sintering.

A sintering step in which the copper fiber nonwoven fabric is sintered at a temperature equal to or lower than the melting point of the copper fibers in a vacuum or non-oxidizing atmosphere is preferably included for sintering the copper fiber nonwoven fabric. After having gone through the sintering step, the copper fiber nonwoven fabric has lost organic matter, and as a result of bonding at the contact points between copper fibers of a sheet composed only of copper fibers in this manner, more favorable shape followability and cushioning can be imparted to the metal fiber sheet.

Moreover, the homogeneity of the sintered copper fiber nonwoven fabric can be further enhanced by carrying out a pressing (pressurizing) step after sintering. Compressing a copper fiber nonwoven fabric, in which the fibers have been randomly entangled, in the direction of thickness not only causes a shift in the fibers in the direction of thickness, but also in the planar direction. As a result, the effect of facilitating the arrangement of copper fibers even at those locations where there were voids during sintering can be expected to be demonstrated, and such a state can be maintained by a plastic deformation property of copper fibers. As a result, the effect is demonstrated of facilitating the imparting of more accurate shape followability and cushioning to the copper fiber nonwoven fabric with little in-plane variation. Pressure during pressing (pressurizing) is suitably set in consideration of the thickness of the copper fiber nonwoven fabric.

(Applications of Copper Fiber Nonwoven Fabric)

Next, an explanation is provided of applications of the copper fiber nonwoven fabric of the present invention. Although there are no particular limitations on applications of the copper fiber nonwoven fabric of the present invention, it can be used in a wide range of applications such as whole sound transmission materials for providing microphones with wind protection, members providing countermeasures against electromagnetic noise used in electronic circuit boards for the purpose of suppressing electromagnetic waves, copper fiber nonwoven fabric heat transfer materials used as measures against heat generation by semiconductors when soldering to connect semiconductor chips, as well as in applications for providing countermeasures against heat dissipation, heat generation or electromagnetic waves such as in construction materials, vehicles, aircraft and marine members.

EXAMPLES Example 1

Copper fibers having a fiber diameter of 18.5 μm, fiber length of 10 mm and a roughly circular cross-sectional shape were dispersed in water followed by the suitable addition of a thickener to obtain papermaking slurry. This papermaking slurry was loaded onto a papermaking mesh based on a basis weight of 300 g/m² followed by dehydrating and drying to obtain a copper fiber nonwoven fabric. Subsequently, after pressing the same nonwoven fabric at a linear pressure of 240 kg/cm at normal temperature, the nonwoven fabric was heated for 40 minutes at 1020° C. in an atmosphere consisting of 75% hydrogen gas and 25% nitrogen gas to obtain the copper fiber nonwoven fabric of Example 1. The thickness of the resulting copper fiber nonwoven fabric was 166.9 μm and the space factor was 19.4%.

Graphs obtained during measurement of the relationship between compressive stress and strain in the copper fiber nonwoven fabric of Example 1 are shown in FIGS. 3 and 4.

Example 2

The copper fiber nonwoven fabric of Example 2 was obtained in the same manner as Example 1 with the exception of adjusting the thickness to 213.8 μm and the space factor to 15.8%.

A graph obtained during measurement of the relationship between compressive stress and strain in the copper fiber nonwoven fabric of Example 2 is shown in FIG. 5.

Example 3

The copper fiber nonwoven fabric of Example 3 was obtained in the same manner as Example 1 with the exception of adjusting the thickness to 332.8 μm and the space factor to 10.3%.

A graph obtained during measurement of the relationship between compressive stress and strain in the copper fiber nonwoven fabric of Example 3 is shown in FIG. 6.

Example 4

The copper fiber nonwoven fabric of Example 4 was obtained in the same manner as Example 1 with the exception of using copper fibers having a fiber diameter of 30 μm and fiber length of 10 mm and adjusting the thickness to 149 μm and the space factor to 24%.

A graph obtained during measurement of the relationship between compressive stress and strain in the copper fiber nonwoven fabric of Example 4 is shown in FIG. 7.

Example 5

The copper fiber nonwoven fabric of Example 5 was obtained in the same manner as Example 1 with the exception of using copper fibers having a fiber diameter of 40 μm and fiber length of 10 mm and adjusting the thickness to 177 μm and the space factor to 18.8%.

A graph obtained during measurement of the relationship between compressive stress and strain in the copper fiber nonwoven fabric of Example 5 is shown in FIG. 8.

Example 6

The copper fiber nonwoven fabric of Example 6 was obtained in the same manner as Example 1 with the exception of using copper fibers having a fiber diameter of 50 μm and fiber length of 10 mm and adjusting the thickness to 179 μm and the space factor to 20.4%.

A graph obtained during measurement of the relationship between compressive stress and strain in the copper fiber nonwoven fabric of Example 6 is shown in FIG. 9.

Comparative Example 1

Surface-roughened copper foil having a thickness of 40 μm was prepared. The space factor of this copper foil was 80%.

A graph obtained during measurement of the relationship between compressive stress and strain in the copper foil of Comparative Example 1 is shown in FIG. 10.

(Measurement of Sheet Thickness)

The thicknesses of the copper fiber nonwoven fabrics obtained in the examples and the copper foil obtained in the comparative example were measured with a probe having a diameter of 15 mm using the ID-C112X Digimatic Indicator manufactured by Mitutoyo Corp. The thicknesses of the resulting sheets were measured at 9 points and the average value thereof was taken to be the thickness of the sheet.

(Measurement of Sheet Dimensions)

The dimensions of the short side and long side of the copper fiber nonwoven fabrics obtained in the examples and copper foil obtained in the comparative example were measured using a JIS first class metal ruler.

(Space Factor)

Space factor of the copper fiber nonwoven fabrics obtained in the examples was calculated in the manner indicated below.

Space factor (%)=(basis weight of copper fiber nonwoven fabric/thickness of copper fiber nonwoven fabric×true density of copper fibers)×100

Space factor of the copper foil obtained in the comparative example was calculated in the manner indicated below.

Space factor (%)=bulk density of the sheet/true specific gravity of the material×100

(Confirmation of Plastic/Elastic Deformation)

The relationship between compressive stress and strain of the sheet-like materials (such as copper fiber nonwoven fabrics) prepared in the examples and comparative examples was measured according to the method indicated below.

The aforementioned sheet-like materials were cut into squares measuring 30 mm on a side followed by placing in a tension-compression strain measuring tester (trade name: RTC-1210A, A & D Co., Ltd.). The thickness of the sheet-like material prior to compression testing is the value obtained by measuring using the aforementioned method for measuring sheet thickness. Since the sheet-like material has the potential for being crushed even by a small amount of stress, when lowering the measuring probe, lower as slowly as possible so that only the dead weight of the probe acts on the sheet-like material. Moreover, the probe is contacted with the sheet-like material only once. The thickness measured in this manner is taken to be the “pre-test thickness”.

Continuing, a compression test is carried out using the sheet-like materials. A 1 kN load cell is used. A compression probe made of stainless steel and having a diameter of 100 mm is used for the jig used in the compression test. The compression rate is set to 1 mm/min and the compression and release operations are consecutively carried out three times on the sheet-like material. As a result, plastic deformation and elastic deformation of the sheet-like material can be confirmed. Actual strain in response to stress can be calculated from a “stress-strain curve chart” obtained from testing, and the amount of plastic deformation and plastic deformation rate can be calculated in accordance with the equation below.

Amount of plastic deformation (μm)=T0−T1

Plastic deformation rate (%)=(T0−T1)/T0×100

The aforementioned T0 refers to the thickness of the copper fiber nonwoven fabric prior to applying the load, while the aforementioned T1 refers to the thickness of the copper fiber nonwoven fabric after releasing the load after it has been applied.

Thickness of the test piece after testing is measured using the same method as previously described and this is taken to be the “post-test thickness”. Thickness after testing was measured 3 hours following completion of testing.

(Measurement of Elongation Percentage and Tensile Strength)

Area of the test piece was adjusted to 15 mm×180 mm in compliance with JIS P8113 followed by measuring the elongation percentage and tensile strength of the copper fiber nonwoven fabrics of the examples and copper foil of the comparative example at a pulling speed of 30 mm/min.

(Pressing Test)

Shape followability and cushioning of the sheet-like materials (such as copper fiber nonwoven fabrics) prepared in the examples and comparative example were confirmed according to the method indicated below.

The aforementioned sheet-like materials were cut to 100 mm×150 mm followed by superimposing an SUS plate 9 (thickness: 1 mm), elastic body 11 (thickness: 1 mm), mirrored SUS plate 10 (thickness: 1 mm), resin sheet 12 (thickness: 0.97 mm), sheet-like material 13 (such as a copper fiber nonwoven fabric), pressure-sensitivity sheet 14 (thickness: 0.18 mm), mirrored SUS plate 10 (thickness: 1 mm), elastic body 11 (thickness: 1 mm) and SUS plate 9 (thickness: 1 mm) in that order starting from the top to form a compressed body 7 as shown in FIG. 12. Next, the compressed body is placed between a stationary plate 6 and a movable plate 8 of the pressing device 3 shown in FIG. 11 provided with a power supply 4 and a heater 5, and after hot-pressing under conditions of 120° C. and 0.5 MPa for 2 minutes, cold pressing was carried out under conditions of 0.5 MPa for 3 minutes. Subsequently, the pressure was released and the degree of discoloration of the pressure-sensitive sheet 14 was observed and evaluated.

A pre-scale ultra-low pressure sheet (two-sheet type, Fujifilm Corp.) was used for the pressure-sensitive sheet 14.

Evaluation criteria were as indicated below.

⊚: Hardly any white areas where pressure is not sensed

◯: Slight appearance of white areas indicative of low sensitivity but no clear difference in in-plane pressure sensitivity

X: Prominent presence of white areas indicative of low sensitivity and prominent difference in in-plane pressure sensitivity

FIG. 13 is a photomicrograph of the pressure-sensitive sheet of Example 1 after the press test, FIG. 14 depicts that of Example 5 and FIG. 15 depicts that of Comparative Example 1.

(Clark Stiffness Test)

Shape followability of the sheet-like materials (such as copper fiber nonwoven fabrics) of the examples and comparative example was confirmed with the Clark Stiffness Tester Method of JIS P 8143:2009.

The presence or absence of plastic deformation and elastic deformation, results of the press test and Clark stiffness and the like of the copper fiber nonwoven fabrics of the examples and copper foil of the comparative example are shown in Table 1.

Physical property values and the like of the copper fiber nonwoven fabrics of the examples and copper foil of the comparative example are shown in Table 2.

TABLE 1 Presence or absence of plastic deformation/ Region exhibiting elastic elastic deformation deformation First Second Presence region region Presence or absence Presence exhibiting exhibiting or of or Press plastic elastic absence inflection absence test Clark deformation deformation of B1 portion of B2 results stiffness Ex. 1 ◯ ◯ ◯ ◯ ◯ ⊚ 6.5 Ex. 2 ◯ ◯ ◯ ◯ ◯ ⊚ 7.3 Ex. 3 ◯ ◯ ◯ ◯ ◯ ⊚ 11.1 Ex. 4 ◯ ◯ ◯ ◯ ◯ ⊚ 6.3 Ex. 5 ◯ ◯ ◯ ◯ ◯ ◯ 5.9 Ex. 6 ◯ ◯ ◯ ◯ ◯ ◯ 3.4 Comp. X ◯ X X ◯ X 17.7 Ex. 1

TABLE 2 Amount Avg. Avg. of fiber fiber Space plastic Plastic Elongation Tensile diameter length Thickness factor deformation deformation percentage strength (μm) (mm) (μm) (%) (μm) rate (%) (%) (N/10 mm) Ex. 1 18.5 10 166.9 19.4 87 47.9 8.2 7.7 Ex. 2 18.5 10 213.8 15.8 125 53.9 9.5 7.7 Ex. 3 18.5 10 332.8 10.3 163 59.7 9.2 8 Ex. 4 30 10 149 24 86 57.7 5.4 6.4 Ex. 5 40 10 177 18.8 80 46.2 4 3.5 Ex. 6 50 10 179 20.4 50 33.5 3.6 2.9 Comp. — — 40 80 0 0 1.5 103.7 Ex. 1

As has been described above, the copper fiber nonwoven fabric of the present invention was determined to have the effect of equalizing differences in in-plane pressure based on the results of press testing as shown in the examples, and have greater ductility than copper foil according to the Clark stiffness tester method. Namely, the copper fiber nonwoven fabric of the present invention, in the relationship between compressive stress and strain, has a first region exhibiting plastic deformation and a second region exhibiting elastic deformation in which compressive stress is higher than in the first region, or has a region exhibiting elastic deformation and the region exhibiting elastic deformation has an elastic deformation region before an inflection portion a, the inflection portion a, and an elastic deformation region after the inflection portion a. As a result, it is able to demonstrate high shape followability and cushioning.

INDUSTRIAL APPLICABILITY

A copper fiber nonwoven fabric can be provided that has cushioning while also have high shape followability.

BRIEF DESCRIPTION OF THE REFERENCE SYMBOLS

1 Copper fiber

2 Bonding portion

A First region exhibiting plastic deformation

B Second region exhibiting elastic deformation (region exhibiting elastic deformation)

B1 Elastic deformation region before inflection portion a

B2 Elastic deformation region after inflection portion a

a Inflection portion

3 Pressing device

4 Power supply

5 Heater

6 Stationary plate

7 Compressed body

8 Movable plate

9 SUS plate

10 Mirrored SUS plate

11 Elastic body

12 Resin sheet

13 Sheet-like material

14 Pressure-sensitive sheet 

1. A copper fiber nonwoven fabric having a bonding portion between copper fibers and, in the relationship between compressive stress and strain, is provided with a first region exhibiting plastic deformation and, a second region exhibiting elastic deformation in which compressive stress is higher than the first region.
 2. The copper fiber nonwoven fabric according to claim 1, having an inflection portion a in the second region exhibiting elastic deformation where strain exhibits inflection in response to compressive stress.
 3. A copper fiber nonwoven fabric having a bonding portion between copper fibers and, in the relationship between compressive stress and strain, has a region exhibiting elastic deformation, wherein the region exhibiting elastic deformation has an elastic deformation region before an inflection portion a, the inflection portion a, and an elastic deformation region after the inflection portion a. 